The exposure setting of a camera determines the amount of light that enters the camera through the lens when taking a photograph, and is adjusted by changing the shutter speed and/or the aperture size of the camera. Most digital cameras include a Through-The-Lens (TTL) light meter that measures the intensity of the light entering the camera through the lens. The light meter reading can be used either automatically by the microprocessor of the camera, or manually by the photographer, to determine the appropriate exposure setting for the particular lighting conditions before a photograph is taken.
Most digital cameras include an automatic mode in which the exposure is set according to the intensity of light that is detected by the light meter before a photograph is taken. In order to determine the appropriate exposure setting, the camera assumes that the scene being photographed has a total visible light reflectance of 18% (ie the scene reflects 18% of incident visible light). Clearly, however, exposure problems can occur when the scene being photographed has a total visible light reflectance that is significantly different to the assumed value of 18%. For example, a scene including large expanses of snow and sky will cause the camera to set the exposure too low, and hence the resulting photograph will be too dark.
One method of setting the exposure more accurately involves using a so-called “grey card”. A grey card has a grey target surface with a visible light reflectance of 18%. In use, the photographer positions the grey card close to the subject being photographed. The photographer then causes the camera to take a reading of the intensity of visible light being reflected by the grey card, and the camera then determines the exposure appropriate for the particular lighting conditions. The photographer is then able to manually adjust the exposure appropriately before taking a photograph.
Most digital cameras also include means for setting white balance. This is because the relative intensities of different wavelengths of visible light received by the camera from a particular subject will depend upon the light source(s) illuminating that subject, and in particular the wavelengths of visible light that are emitted by the light source(s). The white balance setting of a camera is used to alter the colours recorded in the photograph so as to obtain a photograph having a more natural-looking set of colours. Most digital cameras have an automatic mode in which the white balance is set automatically when taking a photograph according to the relative intensities of different wavelengths of visible light that are detected by the camera. In this automatic mode, however, the camera assumes that the scene being photographed reflects the same proportion of each wavelength of visible light falling upon it, and hence that the scene has an overall grey colour.
One method of setting the white balance more accurately involves using a white or grey surface to set the white balance of the camera appropriately for the particular lighting conditions. A white surface is often provided on the rear surface of a grey card for convenience. In use, the white or grey surface is placed close to the subject to be shot, and the photographer causes the camera to set the white balance to an appropriate setting using the white or grey surface as a reference.
Although the use of grey cards and white or grey surfaces to calibrate the exposure and white balance settings of cameras is effective, the process can be time-consuming and confusing for an amateur photographer.
There has now been devised an improved accessory for photographic digital cameras and an improved method of taking a photograph which overcome or substantially mitigate the above-mentioned and/or other disadvantages associated with the prior art.